With the discovery of conducting polymers in the late 1970s, the possibility of combining the important electronic and optical properties of semiconductors and metals with the attractive mechanical properties and processing advantages of polymers was proposed Without exception, however, the initial conducting polymer systems were insoluble, intractable, and nonmelting (and thus not processable) with relatively poor mechanical properties.
In recent years, progress has been made toward rendering specific conjugated polymer systems soluble and thereby processable. For example, the poly(3-alkylthiophene) derivatives (P3ATs) of polythiophene are soluble and meltable with alkyl chains of sufficient length, and the P3ATs have been processed into films and fibers. See, e.g., Hotta, S., et al., Macromolecules, 20:212 (1987); Nowak, M., et al., Macromolecules, 22:2917 (1989); Eisenbaumer, R.L., et al., Synth. Met., 26:267 (1988). However, because of the moderate molecular weights and/or the molecular structures of these polymers, the mechanical properties (modulus and tensile strength) of fibers and films, etc., of the P3ATs are modest and limit their use in applications.
Alternative methods of processing conductive polymers have been developed. For example, poly(phenylenevinylene) ("PPV") and alkoxy derivative of PPV have been synthesized via the precursor polymer route. See, for example, U.S. Pat. Nos. 3,401,152 and 3,706,677 to Wessling et al.; Gagnon et al., Am. Chem. Soc Polym. Prepr. 25:284 (1984); Momii et al., Chem. Lett. 7:1201-4 (1988); and Yamada et al., JCS Chem. Commun. 19:1448-9 (1987). In the first step, a saturated precursor polymer is synthesized; the precursor polymer is soluble and can be processed into the desired final shape. As the final step, the precursor polymer is thermally converted into the desired final shape. As the final step, the precursor polymer is thermally converted to the conjugated polymer. Since tensile drawing can be carried out during the thermal conversion heat treatment, significant chain extension and chain alignment of the resulting conjugated polymers can be achieved. Although the precursor polymer route may offer advantages, the multi-step synthesis is complex, makes the resultant materials relatively expensive, and limits their utility.
On the other hand, ultra-high molecular weight ("UHMW") polyethylene ("PE") can be chain-extended and chain-aligned by first dissolving the polymer in an appropriate solvent at an elevated temperature, then forming a gel by cooling, and subsequently carrying out tensile drawing at selected conditions (temperature, time, etc.) to yield fibers and films etc. with the truly outstanding mechanical properties which characterize high-performance polymers (see Table 1 below).
Today, most polymers (such as UHMW PE) with outstanding mechanical properties are insulators. It would clearly be desirable to render such materials conducting. Previous attempts to render such materials conducting have utilized the general method of filling them with a volume fraction of conducting material such as particles of carbon black, or metal flakes or particles (for example, silver flakes). Addition of such fillers at sufficiently high quantity to yield connected conducting paths (i.e., to be above the percolation threshold; for example, typically about 16% v/v for approximately spherical particles) results in moderate electrical conductivity, but at the expense of the material's mechanical properties. That is, tensile strength and elongation at break are severely reduced by incorporation of the fillers.
Similarly, most polymers with outstanding mechanical properties are either not readily dyed, or, when pigmented, exhibit some loss of tensile strength or modulus, or both.
Thus, the ability to obtain conducting and/or colored olefins or other polymers with attractive mechanical properties and the ability to fabricate such conductive polymers into shaped articles such as fibers, films and the like remains seriously limited.